UMass Amherst CHEM 265 - Recrystallization (9 pages)

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Recrystallization



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Recrystallization

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Pages:
9
School:
University of Massachusetts Amherst
Course:
Chem 265 - Organic Chem-Maj
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Recrystallization 1 Recrystallization is the most convenient method for purifying organic compounds that are solids at room temperature Compounds obtained from natural sources or from reaction mixtures almost always contain impurities The impurities may include some combination of insoluble soluble and colored impurities To obtain a pure compound these impurities must be removed Each is removed in a separate step in the recrystallization procedure To understand the recrystallization process solubility behavior must first be considered It is often stated that like dissolves like More correctly it should be stated that compounds having similar structural features will be soluble in one another Some obvious structural features that may affect solubility include polarity and the ability to hydrogen bond For example a compound having just a few carbons and an alcohol functional group FG would be expected to be soluble in solvents that have a few carbons and an alcohol FG or in some other polar solvent and to be less soluble in nonpolar solvents Conversely an alkane would be expected to show the opposite solubility behavior In most cases though it is not as simple as this If for example a compound has lots of carbons and hydrogens 6 C s and just one alcohol group the solubility will be dominated more by the alkyl part of the molecule than by the alcohol part and the compound will show a solubility behavior more like that of an alkane For known compounds it is useful to consider the structure of the compound when choosing a recrystallization solvent An educated guess can save some time Usually however the structure of a compound may not be known so the solvent must be chosen by carrying out solubility tests The first part of this experiment involves carrying out solubility tests on known compounds Later on such solubility tests will be used to find a suitable recrystallization solvent for an unknown compound A compound usually exhibits one of three general solubility behaviors 1 the compound has a high solubility in both hot and cold solvent green 2 the compound has a low solubility in both hot and cold solvent purple and 3 the compound has a high solubility in hot solvent and a low solubility in cold solvent red Solvents which exhibit the first two behaviors are not useful for recrystallizing a compound A solvent showing the third behavior that is high solubility at high temperatures sparingly soluble at room temperature and low solubility or insoluble at reduced temperatures is one that is suitable for use as a recrystallization solvent Increasing Solubility Good high soluble at high temp low solubility at low temps Bad too soluble at all temperatures Bad too insoluble at all temperatures Increasing Temperature 1 revised 5 30 2018 1 Consider the three different types of impurities that may be present in a sample soluble insoluble and colored In theory insoluble impurities can be removed from a compound fairly easily The compound is dissolved in a solvent the solution is filtered to remove the insoluble impurities and the solvent evaporated to produce the solid compound The insoluble impurities are left behind in the filter paper Colored impurities can be removed in a similar way but with an additional step The solid is dissolved in a solvent activated charcoal is added the solution is filtered as before and the solvent is evaporated to produce the solid compound The charcoal which has adsorbed the colored impurities is left behind in the filter paper The third type of impurity the soluble impurity cannot be filtered out because it has solubility characteristics similar to those of the desired compound hence the name soluble impurity To remove soluble impurities first by doing solubility tests a suitable solvent is chosen high solubility in hot solvent low solubility in cold solvent The soluble impurities are then removed as follows the desired compound along with the soluble impurities are dissolved in a MINIMUM of near or at BOILING solvent The solution is then allowed to cool slowly and without interruption As the solution cools the solubility of the compound and of the soluble impurities decreases the solution becomes saturated with the desired compound and the compound begins to crystallize Because formation of crystals is a highly selective process that usually excludes foreign molecules only crystals of the desired compound form Because the soluble impurities are present in smaller amounts the solution never becomes saturated with the impurities so the impurities remain in solution even after the solution has cooled Removing the solution from the crystals thus removes the solvent and the soluble impurities from the desired crystals A final rinse with a MINIMUM of ICECOLD solvent followed by its removal cleans off any residual soluble impurities clinging to the surface of the desired crystals After allowing the solvent to evaporate pure crystals of desired compound should remain The mass and the MP of the crystals would be determined and along with the recovery and this data would be included in the report In practice by following a set procedure the same solvent is used throughout the entire recrystallization process and the impurities are removed one by one Note that in any recrystallization some of the desired product is sacrificed and the recovery will be less than 100 This is because even at the lower temperatures the desired compound has some finite solubility in the recrystallization solvent and is thus lost when solvent and soluble impurities are removed To illustrate this look at the solubility of acetanilide in water H N O Solubility in 100 mL water 0 5 g at 10 C 5 g at 99 C Acetanilide If 5 g were dissolved in 100 mL of water at 99 C and then allowed to cool to 10 C 4 5 g would crystallize out and 0 5 g would remain in solution The recovery then would be 90 The 10 remaining in solution would be lost Recrystallization summary For our recrystallizations you may assume that only soluble impurities are present The six steps used here to recrystallize a compound are 1 carry out solubility tests to determine a suitable solvent 2 dissolve the solute in a minimum of near or at boiling solvent 3 allow the solution to cool slowly and undisturbed to room temperature rt then possibly to ice temperature 4 collect the crystals by filtration 5 rinse the crystals with a minimum amount of ice cold solvent and 6 allow the crystals to dry 2 What can go wrong in a recrystallization Care


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